Accompanying recent popularization of computers, an inkjet printer is widely used for printing letters or an image on paper, film, cloth or the like not only in offices but also at homes.
The inkjet recording method includes a system of jetting out a liquid droplet by applying a pressure using a piezo element, a system of jetting out a liquid droplet by generating bubbles in an ink under heat, a system of using an ultrasonic wave, and a system of jetting out a liquid droplet by suction using an electrostatic force. The ink used for such inkjet recording includes an aqueous ink, an oily ink and a solid (fusion-type) ink. Among these inks, an aqueous ink is predominating in view of production, handleability, odor, safety and the like.
The coloring agent used in such an ink for inkjet recording is required to have high solubility in a solvent, enable high-density recording, provide good color, exhibit excellent fastness to light, heat, air, water and chemicals, ensure good fixing on an image-receiving material and less bleeding, give an ink having excellent storability, have high purity and no toxicity, and be available at a low cost. However, it is very difficult to find out a coloring agent satisfying these requirements in a high level. Various dyes and pigments for inkjet use have been already proposed and actually used, however, a coloring agent satisfying all requirements is not yet found at present. Conventionally well-known dyes and pigments having a color index (C.I.) number can hardly satisfy both color hue and fastness required of the ink for inkjet recording and a coloring agent for inkjet use having excellent color hue and satisfied in the fastness has not yet been obtained.
As the color image-forming agent having excellent durability, a pigment is generally superior to a dye, however, in the case of a dispersion-type ink using a pigment, the image is inferior in the transparency (particularly, in the high-density area) or image quality. Therefore, neither a dye nor a pigment can satisfy both the fastness and the high image quality at the same time. In recent years, a problem arises that the image formed by the inkjet system is readily discolored particularly by ozone present in the environmental atmosphere and even a phthalocyanine dye known to be a fast dye is discolored. This discoloration reaction greatly affects the color balance of the image. In particular, when a reflection image is formed, discoloration due to ozone proceeds in aging during storage and since the proceeding of discoloration differs every each coloring agent constituting the image, this gives rise to a problem that with the progress of discoloration (reduction in the color density), the image loses the color balance.
Such deterioration in the image quality of the recorded image with the passage of time is accelerated, for example, by high temperature, high humidity, illumination in high brightness or exposure to an oxidative atmosphere such as ozone gas. Therefore, although it is of course important that the ink in the ink composition is excellent in all the heat stability, light fastness and oxidation resistance, the above-described disruption of color balance cannot be overcome merely by rendering a coloring agent for a specific ink to be fast.